The following disclosure generally relates to electrical circuits and signal processing.
Some circuit applications require a low output impedance. For these applications, a source follower (sometimes referred to as a common drain) can be used as a buffer to reproduce a signal at a reduced output impedance.
FIG. 1A is a schematic diagram illustrating a conventional source follower 100. Source follower 100 includes an input terminal 101, an NMOS-type MOSFET 110 (i.e., a Metal-Oxide Field-Effect Transistor having an n-type well), a PMOS-type MOSFET 120 (i.e., an MOSFET having an p-type well), and an output terminal 199. Input terminal 101 is coupled to respective gates of MOSFET 110 and MOSFET 120 for receiving an input signal VIN. Output terminal 199 is coupled to respective sources of MOSFET 110 and MOSFET 120 for producing a conventional output signal VCONV. When a voltage level of input signal VIN at input terminal 101 increases, MOSFET 110 and MOSFET 120 increase a voltage at their drains, and consequentially, increase a voltage level of conventional output signal VCONV at output terminal 199. The opposite occurs when the voltage level of the input signal VIN decreases.
FIG. 1B is a graph 150 illustrating output voltage characteristics of source follower 100 relative to the input voltage. Graph 150 includes an x-axis 151 representing time, a y-axis 152 representing voltage, and shows a plot of input signal VIN 155 and conventional output signal VCONV 165. As shown in FIG. 1B, output signal VCONV 165 lags input signal VIN 155 with respect to time. Also, conventional output signal VCONV 165 is clipped by a value Vgs (the gate/source voltage of MOSFET 110) relative to input signal VIN 155.
Conventional source follower performance suffers from many drawbacks. One problem is that applications such as a motor controller require an output signal having a higher slew rate than a conventional source follower, with a lagging output signal, is able to produce. Another problem is that a reduced output swing in an output signal may not be sufficient for many applications.